It is known in the prior art that adsorbents comprising crystalline aluminosilicates may be used for separating certain hydrocarbons from mixtures containing them. In the field of separating aromatic hydrocarbons and in particular separating aromatic C8 isomers, it is generally acknowledged that the use of particular cations in the cationic sites of crystalline zeolite aluminosilicates improves the selectivity of the zeolite for one of the aromatic C8 isomers. This differential adsorption within the zeolite allows separation of the various aromatic C8 isomers, which is used industrially for the production of very pure para-xylene from an aromatic hydrocarbon feed containing isomers bearing 8 carbon atoms.
Thus, the use of zeolite adsorbents constituted of zeolites X or Y comprising, besides sodium cations, barium, potassium or strontium ions, alone or as mixtures, for selectively adsorbing para-xylene in the liquid phase in an aromatic hydrocarbon mixture, is well know in the prior art.
U.S. Pat. Nos. 3,558,730, 3,558,732, 3,626,020, 3,663,638 and 3,960,774 show that zeolite adsorbents comprising aluminosilicates of faujasite (FAU) structure based on sodium and barium or based on sodium, barium and potassium, are effective for separating para-xylene present in aromatic C8 fractions (fractions comprising aromatic hydrocarbons bearing 8 carbon atoms). The above adsorbents are preferably used as adsorption agents in liquid-phase processes, especially of simulated counter-current type, similar to those described in U.S. Pat. No. 2,985,589 and which apply, inter alia, to aromatic C8 fractions.
However, in general, the adsorption properties of zeolites for aromatic hydrocarbons bearing 8 carbon atoms (xylenes and ethylbenzene) vary very finely as a function of the size and shape of the pores and also of the position of the cations within the structure which have an influence both on the electrostatic field present within the zeolite and on the form of the accessible volume in the pores. Other parameters, such as the polarizability of the cations and molecules or the flexibility of the structure, may also have an influence. It is thus extremely difficult to predict theoretically and with precision the adsorption characteristics of a zeolite with respect to aromatic hydrocarbons bearing 8 carbon atoms.
To improve the adsorption selectivity of zeolites having the faujasite structure for aromatic C8 isomers, many studies have mentioned the influence of the Si/Al ratio of the zeolite, the nature of the exchange cations, and also the water content thereof. Similarly, it is very difficult to predict the degree of improvement since these factors exert combined actions on the adsorption characteristics of zeolites. In particular, it is difficult to predict the impact of the relative proportion of barium ions, and potassium in the case of a zeolite of faujasite (FAU) structure based on barium and potassium, and more precisely in the case of a zeolite of faujasite (FAU) structure of type X, based on barium and potassium.
Patent FR 2 903 978 teaches that potassium ions may represent up to ⅓ of the exchangeable sites occupied by barium and potassium, but said patent does not illustrate any adsorbent containing potassium and does not give any teaching that makes it possible to anticipate the impact of potassium on the adsorption selectivities.
U.S. Pat. Nos. 8,283,274 and 8,557,028 describe adsorbents with potassium weight contents of between 0.25% and 0.9% by weight, corresponding to K2O/(BaO+K2O+Na2O) mole ratios of between 1.3% and 4.5%. U.S. Pat. No. 8,557,028 claims adsorbents with potassium weight contents of between 0.9% and 1.5%, equivalent to K2O/(BaO+K2O+Na2O) mole ratios of between 4.5% and 7.5%. The examples of the latter patent show that the productivity and operating costs are improved with adsorbents that have potassium contents of between 0.7% and 1.2% by weight Patent CN 1267185 describes adsorbents with BaO/K2O mole ratios of between 10 and 40 corresponding to K2O/(BaO+K2O+Na2O) mole ratios of between 2.4% and 9.1%.
The recent patent US 2015/0 105 600 describes an adsorbent based on zeolite X, barium and potassium, with a K2O/(BaO+K2O+Na2O) mole ratio of between 15% and 40%.
The synthesis of zeolites leads to crystals (generally in powder form) whose use at the industrial scale is particularly difficult (substantial losses of feed during the manipulations). The agglomerated forms of these crystals are thus preferred, in the form of grains, yarns and other agglomerates, these said forms possibly being obtained by extrusion, pelleting and other agglomeration techniques known to those skilled in the art. These agglomerates do not have the drawbacks inherent in pulverulent materials.
These agglomerates, whether they are in the form of platelets, beads, extrudates or the like, are generally formed from zeolite crystals, which constitute the active element (in the sense of adsorption) and of a binder intended to ensure the cohesion of the crystals in the form of agglomerates and to give them sufficient mechanical strength to withstand the vibrations and movements to which they are subjected during the operations for separation of the isomers of the C8 aromatic fractions. However, the adsorption properties of these agglomerates are obviously reduced relative to the crystal powder, on account of the presence of agglomeration binder that is inert with respect to adsorption. Various means have already been proposed to overcome this drawback of the agglomeration binder being inert with respect to adsorption performance, among which is the transformation of all or at least part of the agglomeration binder into zeolite that is active from the point of view of adsorption. This operation is now well known to those skilled in the art, for example under the name “zeolitization”. To perform this operation easily, zeolitizable binders are used, usually clays belonging to the kaolinite family, and preferably calcined beforehand at temperatures generally between 500° C. and 700° C.
Patent application FR 2 789 914 describes, for example, a process for manufacturing zeolite X agglomerates, with an Si/Al ratio of between 1.15 and 1.5, containing barium and optionally potassium. The agglomerates thus obtained, after zeolitization of the binder, have, from the point of view of adsorption of para-xylene contained in aromatic C8 fractions, improved properties relative to adsorbents prepared from the same amount of zeolite X and binder, but whose binder is not zeolitized.
The important factors that influence the performance of an adsorption separation process especially encompass the adsorption selectivity, the adsorption capacity and the matter transfer kinetics which defines the rate of adsorption and desorption of the various compounds. The adsorbent must thus have good matter transfer properties so as to ensure a sufficient number of theoretical plates to achieve efficient separation of the mixed species, as indicated by Ruthven in the publication entitled “Principles of Adsorption and Adsorption Processes”, John Wiley & Sons, (1984), pages 326 and 407. Ruthven indicates (ibid., page 243) that, in the case of an agglomerated adsorbent, the overall matter transfer depends on the addition of the intra-crystalline diffusional resistance and of the inter-crystalline diffusional resistance. The intra-crystalline diffusional resistance is proportional to the square of the radii of the crystals and inversely proportional to the diffusivity of the intra-crystalline molecules.
The inter-crystalline diffusional resistance (also known as the macropore resistance) is itself proportional to the square of the radii of the agglomerates and inversely proportional to the diffusivity of the molecules in the macropores. For a given zeolite structure, a given agglomerate size and a given operating temperature, the diffusivities are fixed, and the only way of improving the matter transfer consists in reducing the diameter of the crystals. A gain in the overall transfer will thus be obtained by reducing the size of the crystals.
Consequently, a person skilled in the art expects agglomerated zeolite adsorbents that have both good xylene adsorption capacity and good selectivity for para-xylene to have very good xylene separation properties when they are prepared from small zeolite crystals in liquid-phase processes for separating para-xylene contained in aromatic C8 fractions, for example of the simulated counter-current type. It is, however, impossible for a person skilled in the art to define in principle or theoretically and with precision the adsorption characteristics of an FAU zeolite, especially of X type, having a particular barium and potassium composition, with respect to aromatic hydrocarbons bearing 8 carbon atoms.
The object of the present invention is to provide novel adsorbents based on zeolite X comprising barium, potassium and sodium and having a particular composition of barium, potassium and sodium, which is optimum for simultaneously maximizing the production efficiency and minimizing the production costs of the process for separating para-xylene contained in aromatic C8 fractions. The present invention also proposes a process for separating xylenes using an adsorbent based on zeolite X having a particular composition of barium, potassium and sodium, allowing the production of para-xylene in high purity with improved production efficiency starting with an aromatic hydrocarbon feed containing isomers bearing 8 carbon atoms.